remotenames: avoid updating "selectivepullaccessedbookmarks" if unnecessary

Summary:
I encountered a 19s `hg sl` run where 13s were spent on
`remotenames.updateaccessedbookmarks` (8 times).

{F225347899}

The filesystem is unusually slow since the it's nearly full. However, the
updates are not really necessary since I'm not accessing more remote names.
Accessing one bookmark via smartlog rewrites the entire accessed file, which is
O(N^2) and undesirable.

Improve it by skipping writing if nothing has changed. In the future we might
want to optimize it further by:
- Only update (and do ui.log) "accessedbookmarks" at most once per command.
- Potentially drop the "accessedbookmarks" tracking by using "remotenames"
  directly - everything in "remotenames" is accessed.

Reviewed By: markbt

Differential Revision: D19191528

fbshipit-source-id: 46635d4e5c9d0034ace9cdafc1f42a4512aa8774

eden/scm/edenscm/hgext/remotenames.py

@@ -284,11 +284,24 @@ def updateaccessedbookmarks(repo, remotepath, bookmarks):
     if not _trackaccessedbookmarks(repo.ui):
         return
 
+    # Are bookmarks already marked as accessed?
+    existing = set(
+        name
+        for _node, _nametype, oldremote, name in repo._accessedbookmarks
+        if oldremote == remotepath
+    )
+    newdata = set(bookmarks)
+    if existing.issuperset(newdata):
+        # If so, then skip updating the accessed file.
+        # Note: we ignore the "node" portion of the data since it's not
+        # actually used.
+        return
+
     vfs = repo.sharedvfs
 
     totalaccessednames = 0
     with lockmod.lock(vfs, _selectivepullaccessedbookmarkslock):
-        knownbooks = set(_readremotenamesfrom(vfs, _selectivepullaccessedbookmarks))
+        knownbooks = _readremotenamesfrom(vfs, _selectivepullaccessedbookmarks)
 
         with vfs(_selectivepullaccessedbookmarks, "w", atomictemp=True) as f:
             newbookmarks = {}
@@ -312,6 +325,10 @@ def updateaccessedbookmarks(repo, remotepath, bookmarks):
                 totalaccessednames += 1
                 _writesingleremotename(f, remotepath, "bookmarks", rname, node)
 
+        repo._accessedbookmarks = list(
+            _readremotenamesfrom(repo.sharedvfs, _selectivepullaccessedbookmarks)
+        )
+
     # log the number of accessed bookmarks currently tracked
     repo.ui.log("accessedremotenames", accessedremotenames_totalnum=totalaccessednames)
 
@@ -841,6 +858,9 @@ def reposetup(ui, repo):
         return
 
     repo._remotenames = remotenames(repo)
+    repo._accessedbookmarks = list(
+        _readremotenamesfrom(repo.sharedvfs, _selectivepullaccessedbookmarks)
+    )
 
 
 def _tracking(ui):